Conventional thin-layer field effect transistors (TFTS) utilizing silicon semiconductors or compound semiconductors are used in common integrated circuits and in wide-spreading other applications. In particular, the use of TFTs in liquid crystal displays is well known. Nowadays LC displays are making continuous progress toward larger size and more precise definition. The requirement to incorporate a greater number of TFTs corresponding to the number of pixels becomes stronger than ever.
However, ordinary metal based semiconductors used in the art cannot avoid the problem that slight defects are generated in TFTs formed on the screen as a result of treatments including patterning and etching using photoresists during circuitry formation on the substrate. Such treatments impose a certain limit to the effort of reducing the cost of TFT manufacture. This is also true for other flat displays such as plasma displays and organic EL displays when TFTs are used therein.
The recent trend toward larger size and more precise definition poses a propensity to increase the probability of defectiveness in the TFT manufacture. It is thus strongly desired to minimize such TFT defects.
When patterning, etching and other treatments using photoresists are involved, it is difficult to reduce the fabrication cost below a certain limit.
For TFTs with a metal-insulator-semiconductor (MIS) structure, attempts have been made to use organic materials as the insulator and semiconductor, but only a few reports refer to organic insulating materials. For example, JP-A 5-508745 (WO 9201313 or U.S. Pat. No. 5,347,144) describes a device using an insulating organic polymer having a dielectric constant of at least 5 as the insulator layer and a polyconjugated organic compound having a weight average molecular weight of up to 2,000 as the semiconductor layer. The device exerts a field effect and has a carrier mobility of about 10−2 cm2V−1s−1. Since the semiconductor layer is formed by evaporating α-sexithienyl as an organic semiconductor material, treatments including patterning and etching using photoresists are necessary, failing to achieve a cost reduction.
With respect to organic insulating materials, for example, JP-A 5-508745 and JP-A 2005-72528 (U.S. Ser. No. 10/925,986, EP 045255178.8, CN 200410089945.1) describe polyvinyl alcohol and cyanoethylpullulane, which are of the structure having hydroxyl groups in the molecule. If hydroxyl groups are present in the gate insulating film material, there is a likelihood that hydroxyl groups trap electrons in proximity to the interface with the organic semiconductor layer. Then the device exhibits little or no n-type transistor characteristics, or even when exhibits some, has the drawback of a low carrier mobility.
While the number of new applications to which conventional TFTs utilizing silicon semiconductors or compound semiconductors are employed is increasing, the requirements of lower cost and flexibility are imposed on these devices. To comply with such requirements, active research works have been made on organic semiconductors because of possible fabrication of devices having many advantages including low cost and flexibility. The implementation of organic semiconductors into a commercial practice will lead to the development of printable integrated circuits, electronic paper and the like. However, most organic semiconductors exhibit p-type behavior while only C60 and few other materials exhibit n-type behavior.
N-type organic semiconductors are key materials for the establishment of organic electronic devices including p-n junctions.
In general, organic semiconductors are not prone to polarity inversion from p- to n-type because they have so great a band gap as compared with silicon semiconductor that no inversion layer is formed even when the band is deflected by applying an extra gate voltage. Although the inversion layer may be formed by inducing numerous carriers at the interface between the gate insulating film and the organic semiconductor, a high gate voltage can cause dielectric breakdown if a prior art gate insulating film is used. It is thus difficult to induce a sufficient quantity of carriers to provoke polarity inversion.
For instance, Appl. Phys. Lett., Vol. 85, p 3899 (2004) describes to use an aluminum oxide thin film having a high withstand voltage, high dielectric constant and low leakage current as the insulating film and a single crystal as the organic semiconductor. Allegedly the use of a single crystal as the organic semiconductor eliminates the influence of grains and trap level in the semiconductor thin film, and it is thus expected to gain a high mobility. Although the oxide insulator has the advantages of ease of thin film formation and a high dielectric constant, it undesirably has a low withstand voltage due to the essential presence of oxygen vacancies.
Further, JP-A 2006-303453 (U.S. Pat. No. 7,265,380, EP 06251639.8, CN 200610074140.9) discloses a bipolar OFET comprising a certain organic polymer, typically cyanoethylpullulane as the insulator layer material. It exhibits p-type behavior in the normal state. In order for the transistor to develop n-type behavior, however, a polling operation of applying between source and gate electrodes a voltage which is not less than the coercive electric field and not more than the withstand voltage of the polymer must be carried out.